Register operator definitions - the one surface where an operator's
evaluation, its default picker label, and its valueless flag land
together. Merges over the builtin map (and over earlier withOperators
calls), so builtins stay evaluable and individual keys can be
overridden. Label resolution per row is
i18n.operators[key] ?? def.label ?? key.
Registration alone adds no picker entry: expose the key per field via
FilterFieldDef.operators or per editor type via
withDefaultOperators once an editor can serve it.